1. Field of the Invention
The present invention relates to a liquid-crystal compound having an oxetanyl group as a polymerizable functional group, to a composition containing the compound, to a polymer obtained by polymerizing the compound or the composition, and to their use.
The term “liquid-crystal” as referred to herein is not limited to the meaning that a compound has a liquid-crystal phase. The term may apply also to compounds not having a liquid-crystal phase by themselves but usable as a component of a liquid-crystal composition when mixed with any other liquid-crystal compound.
2. Description of the Related Art
When a polymerizable liquid-crystal compound oriented in a nematic condition is polymerized, then its orientation condition is fixed and it gives an optically-anisotropic film. The optically-anisotropic film is usable as an optically-compensatory film for liquid-crystal display devices. In particular, a shaped article fixed in hybrid-nematic orientation is usable as a viewing angle compensatory element in twisted nematic liquid-crystal display devices. When combined with a polarizer, it is also usable as an elliptically-polarizing plate.
Nematic liquid crystal orientation includes various modes of homogeneous orientation, tilt orientation, homeotropic orientation, hybrid orientation, etc. For example, disclosed is a method of adding an additive such as surfactant to a polymerizable nematic liquid-crystal material, thereby controlling it to hybrid orientation (e.g., see Patent Reference 1). However, a shaped article obtained by curing a composition that contains a non-polymerizable compound such as surfactant added thereto is problematic in that its mechanical strength and heat resistance may lower.
On the other hand, for producing an optically-compensatory film, a polymerizable liquid-crystal composition containing a suitable photopolymerization initiator added thereto is first applied onto a substrate such as rubbed triacetyl cellulose (hereinafter abbreviated to as TAC) films. Next, the liquid-crystal compound in the polymerizable liquid-crystal composition is oriented, and then polymerized through irradiation with electron rays such as UV rays to obtain an optically-anisotropic film in which the liquid crystal orientation is fixed (e.g., see Patent Reference 2).
Proposed are a branched liquid-crystal polyoxetane compound, a liquid-crystal polymer useful in liquid-crystal devices, sensors and non-linear optical devices, and an oxetane monomer for obtaining the polymer. The oxetane monomer has an improved orientation characteristic, a rapid switching speed, and a broad temperature range for liquid-crystal phase (e.g., see Patent Reference 3).
In a process of film production, the step of monomer polymerization through irradiation with UV rays requires the following three conditions from the viewpoint of the equipment and the apparatus for it.
1) Room-temperature polymerization not requiring any specific heating device.
2) Open-air polymerization not requiring purging with inert gas such as nitrogen.
3) Polymerization through irradiation with UV rays at a relatively low total quantity of light.
When a conventional acrylic polymerizable liquid-crystal material is formed into an optically-anisotropic film according to the above-mentioned process, then it brings about the following two problems.
1) In the mode of open-air UV polymerization, oxygen in air retards the polymerization, and a polymer film having good reliability such as heat resistance is difficult to obtain.
2) Since its adhesiveness to the film substrate, TAC is poor, the optically-anisotropic film obtained through polymerization peels from TAC.
A polymerizable liquid-crystal compound and a composition capable of solving these problems are desired.    Patent Reference 1: JP-A 2001-55573    Patent Reference 2: JP-A 2001-154019    Patent Reference 3: JP-T 2003-513107 (U.S. Pat. No. 6,666,989)(The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.)
An object of the invention is to provide a liquid-crystal compound that exhibits nematic hybrid orientation on a rubbed TAC substrate, and a liquid-crystal composition containing it. The film formed by photopolymerizing the compound or the composition keeps the nematic hybrid orientation. Another object of the invention is to provide a liquid-crystal compound having good polymerizability in open air and capable of readily giving a polymer that has a high degree of polymerization even at a relatively small total quantity of light applied thereto, and to provide a liquid-crystal composition that contains the compound. Still another object of the invention is to provide a shaped article having good adhesiveness to a TAC substrate.